The genes for green/grey/blue eyes would have already been there for it to be selected. In northern latitudes, dark skin was probably a disadvantage with regards to vitamin D formation and probably caused all sorts of health problems/disadvantages, and those born with lighter skin were adapted better and, as such, healthier/more competitive for resources and mating opportunities but, apparently, the important trigger may have been increasing lactose tolerance brought about by the advent of farming with dairy produce which allowed a dietary source of vitamin D to be consumed. Put those two factors together the selection veered strongly in favour of lighter skin. Farming started about 8000 years ago, hence why I said it would be another 2000 years from Cheddar Man.